1. Field
The disclosure relates generally to a method and apparatus for assembling a structure by joining components that include components of non-metallic material. More particularly, the disclosure relates to a method and fastener apparatus for mechanically-joining components to assemble composite structures for aircraft and other applications.
2. Background
The use of structures comprised of composite materials has grown in popularity in recent years, particularly in such applications as aircraft, where benefits include exceptional durability and maintainability and a significant savings in weight.
Although composite structures are used in both civil and military aircraft, until quite recently, their use has been generally limited to secondary aircraft components and parts. There has been some reluctance to use composite materials for larger aircraft parts and primary structural airframe components.
Composite structures used in aircraft and other applications are often assembled from a plurality of individual composite components. One particular area of concern in connection with the use of assembled composite structures is the need to mechanically-join the composite components in order to assemble the structure. Despite the development of large co-cured composite structures and the continued refinement in bonding techniques, there remains a need to mechanically-join composite components used in an aircraft, particularly those composite components that may have to be removed for rework or replacement at some stage during the life of an aircraft.
Mechanical fasteners have been used for many years to assemble metallic structures used in aircraft applications, and procedures for assembling metallic structures are relatively straightforward. Using mechanical fasteners to assemble structures formed of advanced composite materials, however, requires a significantly different technological approach than when assembling metallic structures. The full advantages of composite materials cannot be achieved unless there is some reliable mechanism for mechanically-joining components formed of composite material.
Assembling composite structures by mechanically-joining components formed of composite materials such as carbon, epoxy, graphite, carbon/aramid, aramid, and glass-reinforced composite materials, using fasteners formed entirely of metallic components, is well-known in the aircraft industry. Metallic fasteners that are often used to assemble composite structures in aircraft include solid rivets, threaded pins, two-piece bolts, and blind fasteners made of Monel™ metal, titanium, stainless steel, and aluminum-alloy materials. Metallic fasteners, however, are not fully satisfactory for joining components formed of composite materials for several reasons.
Initially, although the solid metallic rivet is the simplest fastener type, when conventional solid metallic rivets, such as solid Monel™ rivets, are used to join components formed of composite materials, the rivets can be less than desirable because the rivets tend to radially expand during installation and produce an edgewise pressure on the composite components. Metallic fasteners, such as aluminum-alloy and stainless-steel fasteners, also expand and contract when exposed to temperature extremes, as may be encountered when used in aircraft applications, which is also less than desirable. Particularly when the components are formed of a carbon-fiber composite material, as is commonly used in aircraft applications, the contraction and expansion of metallic fasteners may cause changes in clamping or preloads associated with the fasteners.
Metallic fasteners used to join composite components may also be subjected to the combined effects of composite relaxation, progressive hole wear caused by cocking or prying forces, thermal variations, and the like.
A particularly significant problem with the use of metallic fasteners for joining composite components is that of galvanic corrosion. Galvanic corrosion may occur when metallic materials, especially aluminum-alloy material, are in contact with composite materials, particularly carbon-fiber composite materials. Galvanic corrosion may be due to chemical reaction of the aluminum with the carbon fibers of the composite components being joined. Although it may be known to apply a sacrificial or protective coating to conventional metallic fasteners to help guard against galvanic corrosion, the coating increases the cost of the fasteners. Fasteners formed of titanium, stainless-steel, or Monel™ materials are better able to resist the problem of galvanic corrosion and may be used instead of aluminum-alloy fasteners to join carbon-fiber composite components. Such fasteners, however, are more expensive than aluminum-alloy fasteners.
In order to prevent contact with the carbon fibers in carbon-fiber composite materials, fastener manufacturers have also tried using various material combinations, including steel and aluminum-alloy fasteners with glass fiber or adhesive-scrim insulation. These material combinations are also not fully satisfactory.
Manufacturers have also experimented with mechanical fasteners formed of composite materials rather than metal. For example, glass or carbon epoxy fasteners are known. Fasteners formed of composite materials, however, are not fully satisfactory in applications such as aircraft applications because they may not provide or achieve appropriate strength and material compatibility characteristics, or meet electrical conductivity requirements.
In particular, aircraft structures must provide a mechanism for dissipating electrical energy, for example, electrical energy generated as a result of the aircraft being struck by lightning. Composite structures used in aircraft, accordingly, typically include an electrically-conductive metallic component that may be sandwiched between assembled composite components or provided as a layer of the composite components to facilitate and satisfy electrical discharge requirements by directing electrical current toward external boundaries of the aircraft, such as wing tips.
If composite fasteners are used to join composite components, however, electric current may be restricted from flowing freely between the joined components; and, as a result, may not provide a suitable path for dissipating electrical current if, for example, the aircraft is struck by lightning. Also, without a suitable path to dissipate electric current, electric potential may build up, and when the electrical potential becomes great enough, a spark or electrical arcing may occur, which may be undesirable for the structure of the aircraft or may cause “noise” in the communications radio or other electrical systems of the aircraft.
There is, accordingly, a need for a mechanism for mechanically-joining components to assemble a composite structure, such as a composite structure of an aircraft, that meets electrical conductivity requirements while providing appropriate strength and material compatibility characteristics.